The present invention relates to a thermo chemical process for removing the binders from metallic and/or ceramic powder injection molded parts. The process uses electrical discharge in a low-pressure hydrogen or another molecular or atomic gas environment to produce reactive species that heat the parts in order to remove the existing binders from inside the parts in a period of time shorter than the one in conventional processes.
The powder metallurgic techniques are very efficient from a technological and economically competitive point of view in the serial manufacturing of parts with complex geometry from iron, steel or other metallic alloys. Within these techniques the injection of metallic and/or ceramic powders with organic binders in molds is one of the most promising. Even so, removing the organic binders after the injection molding and before conducting the final sintering process is a limiting factor. The existent extraction processes demand an excessively long processing time, coming close to dozens of hours thereby, reducing the productivity and raising the price of the product produced by injection molding of powders. Besides demanding an excessively long time, the current binder removal processes produce many pollutants, because they liberate long chain hydrocarbons or they involve burning, which produces residues such as carbon monoxide. The most commonly used binders are mixtures of wax and other polymers, such as, polyethylene, polypylene and polyacetate.
Processes where the plasma technology is used now include, nitrating, carbonitrating, cementation, oxidation and sintering among others.
The process of binder removal by plasma of the present patent application, results in a smaller quantity of gases being emitted. This quantity of gas is essentially short chain hydrocarbons gases, predominantly methane, and can be discharged without any burning with oxygen, thereby avoiding liberation of carbon monoxide.
In the present invention the injected molded part is heat by ions bombarding the cathode. The cathode acts as the injection molded part support. The injection molded part is bombarded together with the cathode and consequently the injection molded part becomes warm. The injection molded part heating happens because the electric discharge of the cathode is connected to the negative voltage of the power source; the ions are strongly accelerated in the cathode sheath electric field straight to the cathode and when hitting the cathode they heat it up. The sintering process by plasma is described for example in PI 9603488-2.
The removal of the binder by the plasma process, herein described, consists of using the plasma technology to produce, besides the heating of the injection molded part, reactive species. These reactive species are fundamental for the kinetics increase of qthe process and cause the removal of the binder to occur in an order of 10 times quicker than in traditional processes. The reactive species are obtained by the electrons of the high kinetic discharge colliding with molecules of hydrogen. The presence of these reactive species is a fundamental difference between the process of the present invention and the other processes of binder removal.